Features

YAML syntax was designed to be easily mapped to data types common to most high-level languages:
list, hash, and scalar. Its familiar indented outline and lean appearance makes it especially suited for tasks where humans are likely to view or edit data structures, such as configuration files, dumping during debugging, and document headers (e.g. the headers found on most e-mails are very close to YAML). Although well-suited for hierarchical data representation, it also has a compact syntax for a relational data as well. Its line and whitespace delimiters make it friendly to ad hocgrep/Python/Perl/Ruby operations. A major part of its accessibility comes from eschewing the use of enclosures like quotation marks, brackets, braces, and open/close-tags which can be hard for the human eye to balance in nested hierarchies.

Examples

Sample document

Data structure hierarchy is maintained by outline indentation.

---
receipt: Oz-Ware Purchase Invoice
date: 2007-08-06
customer:

given: Dorothy

family: Gale

items:

- part_no: A4786

descrip: Water Bucket (Filled)

price: 1.47

quantity: 4

- part_no: E1628

descrip: High Heeled "Ruby" Slippers

price: 100.27

quantity: 1

bill-to: &id001

street: |

123 Tornado Alley

Suite 16

city: East Westville

state: KS

ship-to: *id001

specialDelivery: >

Follow the Yellow Brick

Road to the Emerald City.

Pay no attention to the

man behind the curtain.

...

Notice that strings do not require enclosure in quotations. The specific number of spaces in the indentation is unimportant as long as parallel elements have the same left justification and the hierarchically nested elements are indented further. That sample document defines a hash with 7 top level keys: one of the keys, "items", contains a 2 element array (or "list"), each element of which is itself a hash with four keys. Relational data and redundancy removal are displayed: the "ship-to" hash content is copied from the "bill-to" hash's content as indicated by the anchor(&) and reference(*) labels. Optional blank lines can be added for readability. Multiple documents can exist in a single file/stream and are separated by "---". An optional "..." can be used at the end of a file (useful for signalling an end in streamed communications without closing the pipe).

Language elements

Basic components of YAML

YAML offers both an indented and an "in-line" style for denoting hashes and lists. Here is a sampler of the components.

Lists

Conventional block format uses a dash to begin a new item in list

--- # Favorite movies

- Casablanca

- North by Northwest

- Notorious

Optional inline format is delimited by comma+space and enclosed in brackets (similar to JSON)

--- # Shopping list

[milk, pumpkin pie, eggs, juice]

Hashes

Keys are separated from values by a colon-space.

--- # Block

name: John Smith

age: 33

--- # Inline

{name: John Smith, age: 33}

Block literals

Strings do not require quotation.

Newlines preserved

--- |

There once was a man from Darjeeling

Who got on a bus bound for Ealing

It said on the door

"Please don't spit on the floor"

So he carefully spat on the ceiling

By default, trailing white space is stripped. Use |+ to keep trailing whitespace. Leading whitespace is trimmed to first line's indent. Use |8 to add a leading whitespace indent (where 8 is any number).

Newlines folded

--- >

Wrapped text

will be folded

into a single

paragraph

Blank lines denote

paragraph breaks

Folded text converts newlines to spaces. This behavior cannot be overridden. Use >- to strip white space at the end of the "paragraph" only, where a YAML "paragraph" ends after the last non-empty line.

Hierarchical combinations of elements

Lists of hashes

- {name: John Smith, age: 33}

- name: Mary Smith

age: 27

Hashes of lists

men: [John Smith, Bill Jones]

women:

- Mary Smith

- Susan Williams

Advanced components of YAML

As discussed in a subsequent section, two features that distinguish YAML from the capabilities of other data serialization languages are Relational trees and Data Typing.

Relational trees

Data merge and references

For clarity, compactness, and avoiding data entry errors, YAML provides node references (*) and hash merges (<Merges and references are automatically expanded by the parser when the data structure is instantiated. This can greatly enhance readability and facilitate editing: below is an example of a queue in an instrument sequencer in which each subsequent step only lists the elements that are changed from the first step. When a YAML parser loads this array, all the "step" hashes will have the 5 keys specified in first step.

sequencer protocols for Laser eye surgery

---
- step: &id001 # defines anchor label &id001

instrument: Lasik 2000

pulseEnergy: 5.4

pulseDuration: 12

repetition: 1000

spotSize: 1mm

- step:

<

spotSize: 2mm # overrides "spotSize" key's value

- step:

<

pulseEnergy: 500.0 # overrides key

alert: > # adds additional key

warn patient of

audible pop

Data types

Explicit data typing is seldom seen in the majority of YAML documents since YAML autodetects simple types. Data types can be divided into three categories: core, defined, and user-defined. Core are ones expected to exist in any parser (e.g floats, ints, strings, lists, maps, ...). Many more advanced data types, such as binary data, are defined in the YAML specification but not supported in all implementations. Finally YAML defines a way to extend the data type definitions locally to accommodate user defined classes, structures or primitives (e.g. quad precision floats).

Casting data types

YAML autodetects the datatype of the entity. Sometimes one wants to cast the datatype explicitly. The most common situation is a single word string that looks like a number, boolean or tag may need disambiguation by surrounding it with quotes or use of an explicit datatype tag.

Other specified data types

Not every implementation of YAML has every specification-defined data type. These built-in types use a double exclamation sigil prefix(!!). Particularly interesting ones not shown here are sets, ordered maps, timestamps, and hexadecimal. Here's an example of binary data.

---
picture: !!binary |

R0lGODlhDAAMAIQAAP//9/X

17unp5WZmZgAAAOfn515eXv

Pz7Y6OjuDg4J+fn5OTk6enp

56enmleECcgggoBADs=mZmE

Extension for user-defined data types

Many implementations of YAML can support user defined data types. This is a good way to serialize an object. Local data types are not universal data types but are defined in the application using the YAML parser library. Local data types use a single exclamation mark(!).

Repeated nodes are initially denoted by an ampersand ( & ) and thereafter referenced with an asterisk ( * )

Nodes may be labeled with a type or tag using the exclamation point ( !! ) followed by a string which can be expanded into a URI.

YAML documents in a stream may be preceded by directives composed of a percent sign ( % ) followed by a name and space delimited parameters. Two directives are defined in YAML 1.1:

The %YAML directive is used to identify the version of yaml in a given document.

The %TAG directive is used as a shortcut for URI prefixes. These shortcuts may then be used in node type tags.

YAML requires that colons and commas used as list separators be followed by a space so that scalar values containing embedded punctuation (such as 5,280 or http://www.wikipedia.org) can generally be represented without needing to be enclosed in quotes.

Comparison to other data structure format languages

While YAML shares similarities with JSON, XML and SDL, it also has characteristics that are unique in comparison to many other similar format languages.

JSON

JSON syntax is nearly a subset of YAML and most JSON documents can be parsed by a YAML parser. This is because JSON's semantic structure is equivalent to the optional "inline-style" of writing YAML. While extended hierarchies can be written in inline-style like JSON, this is not a recommended YAML style except when it aids clarity. YAML has additional features lacking in JSON such as extensible data types, relational anchors, strings without quotation marks, and mapping types preserving key order.

XML and SDL

YAML lacks the notion of tag attributes that are found in XML and SDL. For data structure serialization, tag attributes are, arguably, a feature of questionable utility since the separation of data and meta-data adds complexity when represented by the natural data structures (hashes, arrays) in common languages. Instead YAML has extensible type declarations (including class types for objects). YAML itself does not have XML's language-defined document schema descriptors that allow, for example, a document to self validate. However, a YAML schema descriptor language exists, and YAXML, which represents YAML data structures in XML, allows XML schema importers and output mechanisms like XSLT to be applied to YAML. Moreover, in typical use, the semantics provided by rich language-defined type-declarations in the YAML document itself eliminates the need for an additional validator.

Indented delimiting

Because YAML primarily relies on outline indentation for structure, it is especially resistant to delimiter collision. YAML's insensitivity to quotes and braces in scalar values means one may embed XML, SDL, JSON or even YAML documents inside a YAML document by simply indenting it in a block literal:

---
example: >

HTML goes into YAML without modification

message: |

"Three is always greater than

two, even for large values of two"

--Author Unknown

date: 2007-06-01

Conversely, to place YAML in XML or SDL content requires converting all whitespace and potential sigils (like and &) to entity syntax. To place YAML in JSON requires quoting it, and escaping all interior quotes.

Non-hierarchical data models

Unlike SDL, and JSON, which can only represent data in a hierarchical model with each child node having a single parent, YAML also offers a simple relational scheme that allows repeats of identical data to be referenced from two or more points in the tree rather than entered redundantly at those points. This is similar to the facility IDREF built into XML. The YAML parser then expands these references into the fully populated data structures they imply when read in, so whatever program is using the parser does not have to be aware of a relational encoding model, unlike XML processors which do not expand references. This expansion can enhance readability while reducing data entry errors in configuration files or processing protocols where many parameters remain the same in a sequential series of records while only a few vary. An example being that "ship-to" and "bill-to" records in an invoice are nearly always the same data.

Practical considerations

YAML is line oriented and thus it is often simple to convert the unstructured output of existing programs into YAML format while having them retain much of the look of the original document. Because there are no close-tags, braces and quotation marks to balance it is generally easy to generate well-formed YAML directly from distributed print statements within unsophisticated programs. Likewise, the white space delimiters facilitate quick-and-dirty filtering of YAML files using the line oriented commands in grep, awk, perl, ruby, and python.

In particular, unlike mark-up languages, chunks of consecutive YAML lines tend to be well-formed YAML documents themselves. This makes it very easy to write parsers that do not have to process a document in its entirety (e.g. balancing open- and close-tags and navigating quoted and escaped characters) before they begin extracting specific records within. This property is particularly expedient when iterating in a single, stateless pass, over records in a file whose entire data structure is too large to hold in memory, or for which reconstituting the entire structure to extract one item would be prohibitively expensive.

Counterintuitively, although its indented delimiting might seem to complicate deeply nested hierarchies, YAML handles indents as small as a single space, and this may achieve better compression than markup languages. Additionally, extremely deep indentation can be avoided entirely by either: 1) reverting to "inline-style" (i.e JSON-like format) without the indentation; or 2) using relational anchors to unwind the hierarchy to a flat form that the YAML parser will transparently reconstitute into the full data structure.

Security

YAML is purely a data representation language and thus has no executable commands. This means that parsers will be (or at least should be) safe to apply to tainted data without fear of a latent command-injection security hole. For example, because JSON is native JavaScript it's tempting to use the JavaScript interpreter itself to evaluate the data structure into existence, leading to command injection holes when inadequately verified. While safe parsing is inherently possible in any data language, implementation is such a notorious pitfall that YAML's lack of an associated command language may be a relative security benefit.

Data processing and representation

The XML and YAML specifications provide very different logical models for data node representation, processing, and storage.

XML: The primary logical structures in an XML instance document are: 1) Element; and 2) Element attribute. For these primary logical structures, the base XML specification does not define constraints regarding such factors as duplication of elements or the order in which they are allowed to appear. In defining conformance for XML processors, the XML specification generalizes them into two types: 1) validating ; and 2) non-validating. The XML specification asserts no detailed definitions for: an API; processing model; or data representation model; although several are defined in separate specifications that a user or specification implementor may choose independently. These include the Document Object Model and XQuery.

A richer model for defining valid XML content is the W3C XML Schema standard. This allows for full specification of valid XML content and is supported by a wide range of open source, free and commercial processors and libraries.

YAML: The primary logical structures in a YAML instance document are: 1) Scalar; 2) Sequence; and 3) Mapping. The YAML specification also indicates some basic constraints that apply to these primary logical structures. For example, according to the specification, mapping keys do not have an order. In every case where node order is significant, a sequence must be used.

Moreover, in defining conformance for YAML processors, the YAML specification defines two primary operations: 1) Dump; and 2) Load. All YAML-compliant processors must provide at least one of these operations, and may optionally provide both. Finally, the YAML specification defines an information model or "representation graph" which must be created during processing for both Dump and Load operations, although this representation need not be made available to the user through an API.

Implementations

Portability

Simple YAML files (e.g. key value pairs) are readily parsed with regular expressions without resort to a formal YAML parser. YAML emitters and parsers for many popular languages written in the pure native language itself exist, making it portable in a self-contained manner. Bindings to C-libraries also exist when speed is needed.

C libraries

libYAML As of 2007-06, this implementation of YAML 1.1 is stable and recommended by the YAML specification authors for production use (despite the 0.1.1 version number and a mild caution that the API is not barred from evolution.).

SYCK This implementation supports most of YAML 1.0 specification and is in widespread use. It is optimized for use with higher level interpreted languages, obtaining speed by writing directly to the symbol table of the higher level language when it can. As of 2005 it is no longer maintained but remains available.

Pitfalls and implementation defects

An editor mode that autoexpands tabs to spaces and displays text in a fixed-width font is recommended.

The editor needs to handle UTF-8 and UTF-16 correctly (otherwise, it will be necessary to use only ASCII as a subset of UTF-8).

Strings:

YAML allows one to avoid quoted strings which can enhance readability and avoid the need for nested escape sequences. However, this leads to a pitfall when inline strings are ambiguous single words (e.g. digits or boolean words) or when the un-quoted phrase accidentally contains a YAML construct (e.g., a leading exclamation point or a colon-space after a word: "!Caca de vaca!" or "Caution: lions ahead"). This is not an issue that anyone using a proper YAML emitter will confront, but can come up in ad hoc scripts or human editing of files. In such a case a better approach is to use block literals ("|" or ">") rather than inline string expressions as these have no such ambiguities to resolve.

Anticipating implementation idiosyncrasies:

Some implementations of YAML, such as perl's YAML::BASE will load an entire file (stream) and parse it en-mass. Conversely, YAML::Tiny only reads the first document in the stream and stops. Other implementations like pyYaml are lazy and iterate over the next document only upon request. For very large files in which one plans to handle the documents independently, instantiating the entire file before processing may be prohibitive. Thus in YAML::BASE, occasionally one must chunk a file into documents and parse those individually. Fortunately, YAML makes this easy since this simply requires splitting on the document separator, m/^---/. That strategy could be disrupted if anchor and reference tags happen to lie in different documents of the same file.